1. Field of the Invention
This invention relates to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with stimulating rays which cause it to emit light in proportion to the stored radiation energy. This invention particularly relates to a radiation image read-out apparatus which accurately reads out a radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or uItraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals, and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The stimulable phosphor sheet is used as a means for recording mainly a medical image in the aforesaid radiation image recording and reproducing system, and in various other fields. For example, the applicant proposed in Japanese Unexamined Patent Publication Nos. 61(1986)-51738 and 61(1986)-93539 a method of recording and reproducing an electron microscope image by utilizing a stimulable phosphor sheet in an electron microscope. Basically, the proposed method of recording and reproducing an electron microscope image comprises the steps of (i) exposing a stimulable phosphor sheet for storing electron beam energy thereon to an electron beam passing through a sample in a vacuum to have the electron beam energy stored on the stimulable phosphor sheet, (ii) thereafter exposing the stimulable phosphor sheet to, for example, stimulating rays to release the stored energy as light emission, (iii) photoelectrically detecting the emitted light to obtain image signals, and (iv) reproducing an electron beam image of the sample by use of the image signals.
It is desired that the read-out apparatus for irradiating the stimulating rays to the stimulable phosphor sheet and detecting the light emitted by the stimulable phosphor sheet in the aforesaid electron microscope image recording and reproducing method be capable of scanning the stimulating rays at a higher density and detecting the recorded image more accurately than in the read-out apparatus for the aforesaid radiation image recording and reproducing system. However, with the conventional read-out apparatus, it is not always possible to achieve the image read-out at such a higher density and higher accuracy. Specifically, in the radiation image recording and reproducing system, a light beam deflected by a light deflector in the main scanning direction is generally made to impinge upon and converged by a scanning lens such as an f.theta. lens, and is scanned in the main scanning direction on the stimulable phosphor sheet conveyed at a predetermined speed in the sub-scanning direction. In order to minimize the side of the f.theta. lens, an f.theta. lens having a comparatively long focal length must be used and spaced from the stimulable phosphor sheet. In the case where the f.theta. lens having a comparatively long focal length is used, it is not always possible to converge the stimulating rays to a desired beam diameter on the stimulable phosphor sheet. Also, detection of the light emitted by the stimulable phosphor sheet by use of a photodetector in the aforesaid read-out apparatus is carried out via a transparent light guide member having a light input face positioned along the main scanning line and having a rear end shaped to match the photodetector. In this case, the light guiding efficiency of the light guide member readily differs among positions of light emission in the main scanning direction on the stimulable phosphor sheet (this problem is hereinafter referred to as shading), and therefore density nonuniformity arises in the reproduced visible image.
On the other hand, a novel read-out apparatus is disclosed in Japanese Unexamined Patent Publication No. 59(1984)-13235 wherein an optical element such as a dichroic mirror for reflecting or transmitting stimulating rays and transmitting or reflecting light emitted by a stimulable phosphor sheet is provided between a light source and the stimulable phosphor sheet, a condensing element (lens) for converging the stimulating rays coming from the optical element on the stimulable phosphor sheet is provided, and the stimulable phosphor sheet is formed cylindrically and rotated for carrying out scanning in the main scanning direction. In this case, the light emitted by the stimulable phosphor sheet passes through the condensing lens from the direction reverse to the stimulating rays, and is collimated thereby. The collimated light passes through or is reflected by the dichroic mirror, is converged by a converging lens, and is then detected by a photodetector. With the disclosed apparatus, an optical element having a short focal length can be used as the aforesaid condensing element, and the stimulating rays can be converged to a small spot diameter. Also, the light emitted by the stimulable phosphor sheet is condensed by the condensing lens and the converging lens, and therefore no shading arises as in the case where a large light guide member is used.
However, in the radiation image read-out apparatus, the stimulating rays impinging upon the stimulable phosphor sheet are often reflected partially by the stimulable phosphor sheet, and the reflected stimulating rays are again reflected by various components of the read-out apparatus onto portions of the stimulable phosphor sheet, thereby causing the sheet portions to emit light (this phenomenon is hereinafter referred to as the flare phenomenon). When the flare phenomenon arises, the sheet portions outside of the predetermined scanning position of the stimulating rays are caused by the reflected stimulating rays to emit light, and such light is detected by the photodetector together with the light emitted by the predetermined scanning position. Therefore, the reproduced visible image thus obtained becomes incorrect, and contrast of the image deteriorates. It is considered that the reflected stimulating rays impinges upon, for example, the condensing element and is reflected thereby, and the flare phenomenon thus occurs also in the aforesaid read-out apparatus.
Moreover, the aforesaid radiation image read-out apparatus has the drawback that the stimulating rays impinging upon the stimulable phosphor sheet are scattered inside of the sheet, and the scattered stimulating rays cause light emission, thereby to blur the image.